Nickel-cobalt-manganese (NCM) ternary materials have advantages of high specific capacity, low cost and good thermal stability etc., therefore, they have a very broad application prospect in the field of energy storage and electric vehicle. However, poor cycling performance is an important factor affecting the application of NCM ternary materials for a long time. The main transition elements are Ni, Co and Mn in NCM ternary materials, in which NCM ternary materials with different properties can be obtained with different ratios of the three elements. When contacting with an electrolytic solution, the stability of the material also changes due to the different ratios of elements. The factors affecting the cycle life of the ternary materials are: 1. Reconstruction of the surface crystal structure during the cycling process; 2. Secondary particles rupture due to the volume expansion of the anisotropy during the cycling process. It was found that the connecting structure between particle and particle in the secondary particles could cause localized current density to rise, which caused great stress and therefore affected the cycling performance of the material. At the same time, there was a phenomenon of inconsistency of charge state between various parts of particles, which would affect the electrochemical performance of an electrode.
In addition, when the amount of lithium deintercalation is larger, the structure of NCM ternary materials becomes very fragile, and a displacement of active metal and oxygen occurs in the crystal lattice, and when under a certain high temperature and high pressure, atomic rearrangement gradually increases, volume and phase of crystalline grain change largely. On the other hand, chemical and electrochemical reactions of the ternary materials with the electrolyte cause the material easy to deoxidation and the transition metal dissolved, especially the electrolyte in the high-voltage will be oxidized to produce H+ and the acidity of the electrolyte are improved. So a surface film of the electrode material is damaged by HF, and a composition and structure of the interface are further changed, seriously affecting the electrochemical performance and cycling performance of the material.
In order to solve the above-mentioned problems, NCM ternary materials are modified by bulk doping and surface coating modification, which is an effective method. The bulk doping of NCM ternary materials can effectively restrain the structure change, enhance the reversibility of materials and restrain the increase of the charge transfer impedance during the cycling process. The surface coating modification of NCM ternary materials can effectively avoid direct contact of the materials with the electrolyte, especially with HF in the electrolyte, thus prevent the occurrence of side reactions, inhibit crystal phase changes of the materials, thereby improve cycling stability and multiplication of the materials etc.
It was reported in Chinese invention patent of Application No. 200780013946.X, Publication No. CN101427403A, entitled “Positive Electrode Active Material Powder” that, the positive electrode active material powder of this invention comprised primary particles and aggregated particles by primary particles, and the average particle diameter on volume basis of the primary particles and the aggregated particles by the primary particles is 0.1 μm to 3 μm, and the percentage of the sum of the volume of particles having a particle diameter of 5 μm or more to the sum of the volume of all the particles is below 10%, and a BET specific surface area of the powder is more than 2 m2/g and less than 7 m2/g. It was reported in Chinese invention patent of Application No. 200780013765.7, Publication No. CN101421867A, entitled “Positive Electrode Powder and Cathode Mixture” that, the positive electrode powder of this invention contains a positive electrode active material powder and a graphite powder, wherein the positive electrode active material powder contains primary particles and aggregated particles by primary particles, and more than 90% of the particles have a particle diameter of 0.1 μm to 5 μm, and a BET specific surface area of the powder is 1 m2/g to 7 m2/g. It was reported in Chinese invention patent of Application No. 200780004424.3, Publication No. CN101379637A, entitled “A lithium transition metal compound powder of cathode material for lithium secondary battery and preparation methods thereof, a spray-dried body and calcined precursor thereof, a positive electrode for a lithium secondary battery and a lithium secondary battery using the lithium transition metal compound powder” that, the lithium-nickel-manganese-cobalt based composite oxide powder of cathode material for lithium secondary battery in this patent has a half width of the (110) diffraction peak in the vicinity of a diffraction angle 2θ of 64.5° of between 0.01 and 0.2 in the powder X-ray diffraction.